I myself have been researching this and what seems to be popular opinion is two PHs, one on each end of the tank aimed at one another. In addition to this you can add two more lower in the tank to keep the substrate clean. Not everyone does the second two, in fact I have never seen it in anyone's tank but there are those who advocate it in order to keep detritus in the water column and allow it to be filtered out. I have been reading some threads on DSB v. BB and there is a great deal of this discussed in regards to current. There is also the variable of what types of corals you plan to keep. Of course if you are keeping stonies you will want higher flow than if you have softies. I went to my local Aquarium Society meeting this evening and the host had two tanks, one 65G softy tank with one PH and one 100G stony tank with 4 PHs and very high flow. I personally preferred the softy tank which makes me wonder what I will do with my 4 new nanostreams that are supposed to be here tomorrow. I think I may set up a low flow area specifically for my pumping xenias because they look so much nicer under the lower flow. I think it is just a matter of experimentation and preference. I currently have 8 PHs in my 180G but I am not happy with the flow, I think this is mainly because of the narrow stream that is put out by my PHs so I am hoping that the nanostreams will be better. I also purchased 2 Koralia nanos to go on the back wall behind the rock to blow all of the detritus back to the front and I may get a couple more. The nice thing about the K nanos is that they really have a very low flow for a larger tank and they don't cause a sand storm but do move the muck out. I will mod my tunze nanos and let you all know how it goes. I am hoping to get a flip camera for Christmas and I will try to make movie clip.
